In a mobile communications system, the total system coverage is typically deployed by means of many different cells. This is denoted a cellular system. For large coverage areas so-called macrocells are used, providing macro network coverage. Such macro-cellular systems have reached maturity and operators in many countries are reaching the maximum price/performance benefit for this type of infrastructure within their cellular networks. Smaller cells are already being used in certain parts of radio communications systems, providing smaller area coverage and capacity requirements. Such smaller cells may be used to extend coverage in areas where the macrocell network is weak or not available, such as near the cell edges, in urban valleys, inside buildings, etc. In addition, smaller cells may be employed to add spectrum capacity where the macrocell network is congested or overloaded. In such areas, which may be more focused on offering higher system capacity the macro network, smaller cells may thus typically be deployed within areas where macro coverage is already available. Dependent on e.g. the offered cell size, such smaller cells may for example be denoted as microcells, picocells or femtocells. A radio communications network with such a mixed cell size network topology using overlapping cells is sometimes referred to as a heterogeneous network.
Due to increasing capacity demands, a further densification of network cells compared to today can be envisioned. This could result in areas with a significant amount of deployed small cells. One result of such deployment is that a single mobile device located in that area will at certain positions be able to detect signals from base stations of many different cells. The principle of both active mode and idle mode cell selection in many cellular standards, e.g. the 3GPP standards for WCDMA and LTE, are based on mobile device detection of base station signals. Each device will experience the received signals differently, e.g. due to different positions in a cell, and based on the experienced signal quality it may select a suitable cell for camping. More importantly, the device will also provide the serving base station with information to base potential handover decisions on. The base station to which the mobile device is currently connected may be referred to as the serving base station or cell. The serving base station stores, or otherwise has access to, a neighbour cell list. This represents a list of at least the most nearby cell base stations. This cell is transmitted from the serving base station to the mobile device, and at least during active mode each mobile device must report information to the serving base station about each detected cell among the cells in that list.